flag day n.
A software change that is neither forward- nor
backward-compatible, and which is costly to make and costly to
reverse. "Can we install that without causing a flag day for all
users?" This term has nothing to do with the use of the word
flag to mean a variable that has two
values. It came into use when a massive change was made to the
Multics timesharing system to
convert from the short-lived 1965 version of the ASCII code to
the 1967 version (in draft at the time); this was scheduled for
Flag Day (a U.S. holiday), June 14, 1966. The actual change moved
the code point for the ASCII newline character; this required
that all of the Multics source code, documentation, and device
drivers be changed simultaneously. See also backward combatability.